NUSA DUA, Indonesia (Reuters) - U.N. Secretary-General Ban Ki-moon urged the world on Wednesday to agree to work out a new climate treaty by 2009 but said it might be "too ambitious" to set goals for greenhouse gas cuts in Bali.

China said it was disappointed by a lack of progress at the December 3-14 talks and said emissions targets were exactly what was needed to prove rich nations were committed to fight global warming.

Washington is leading opposition at the meeting to any mention of scientific evidence of a need for cuts in greenhouse gases of 25 to 40 percent by 2020 below 1990 levels as part of the guidelines for negotiations.

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"Realistically it may be too ambitious if delegations would be expected to be able to agree on targets of greenhouse gas emission reductions" in Bali, Ban said, echoing a view given by Washington.

"Sometime down the road we will have to agree on them."

Still, he also said that all countries should respect a finding by the U.N. climate panel that a range of 25-40 percent was needed to avert the worst impacts of climate change. The range was still in the draft text on Wednesday evening.

Ban said the overriding goal of the December 3-14 meeting was to agree to launch negotiations on a pact to succeed the current Kyoto Protocol.

He told more than 120 environment ministers that climate change was the "moral challenge of our generation" and said there was a "desperate urgency" to act to curb rising seas, floods, droughts, famines and extinctions of wildlife.

"The time to act is now," Ban told the ministers, split over the ground rules for agreeing to launch formal negotiations on a new long-term global treaty to limit greenhouse gas emissions, expanding the 37-nation Kyoto pact to all countries.

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"You need to set an agenda -- a roadmap to a more secure climate future, coupled with a tight timeline that produces a deal by 2009," he said. The United Nations wants a new pact adopted at a meeting in Copenhagen in late 2009.

The United States, supported by Japan, Canada and Australia, says that even a non-binding mention of a 25-to-40 percent range could prejudge the outcome of negotiations.

Canada's environment minister said the fight against global warming would only work if big developing countries took on legally binding targets, underlining a major split at the talks.

"We are not ripe enough to make any binding commitments. We are a developing country," junior environment minister N.N. Meena said.

A source close to the Chinese delegation said: "The developed countries must take their responsibilities seriously. We must have the 25 to 40 percent target...If they won't agree to this, if they won't even do this, then what hope is there?"

Top oil exporter Saudi Arabia, traditionally wary of shifts towards renewable energies such as wind and solar power that might undermine demand for crude, said the world should make the most of its "huge reserves" of fossil fuels.

Earlier, Australia's new Prime Minister Kevin Rudd handed formal papers to Ban ratifying the Kyoto Protocol, isolating the United States as the only rich nation without binding caps on greenhouse gas emissions under the U.N. deal stretching to 2012.

Rudd said Australia was already suffering from climate change -- ranging from a drying up of rivers to disruptions to corals of the Great Barrier Reef.

The Bali talks are set to end by Friday or early Saturday.

"Some of the reports are worrisome," said joint Nobel Peace Prize winner Al Gore. "But I know from experience and previous such meetings that breakthroughs, when they do occur, usually happen in the last 48 hours and sometimes in the last four to eight hours," he said in Stockholm on his way to Bali.

The United Nations wants a deal in place by the end of 2009 to give parliaments three years to ratify and help guide billions of dollars of investments in everything from solar panels and wind turbines to coal-fired power plants.

A failure of Bali to agree to start talks would sour chances of a successor to Kyoto.